1. Field of the invention
The present invention relates to a liquid droplet ejecting apparatus, and more particularly to the liquid droplet ejecting apparatus applicable as a recording apparatus, a dispenser, or a picture drawing apparatus.
2. Description of the Related Art
Liquid droplet ejecting apparatuses are used for many applications, the most well known being in print heads for ink jet printers. The following explanations relate to ink jet printers.
Japanese Patent Application Kokai No. SHO-48-9622 describes two general apparatuses for ink jet printing wherein ink within an ink channel has applied thereto a pulse of pressure which ejects the ink from a nozzle. In one apparatus, hereinafter referred to as a piezoelectric type ejector, the pulses of pressure are applied by deformation of a piezoelectric element. In the other apparatus, hereinafter referred to as a bubble ink jet apparatus, a heat resistor rapidly heats a portion of the ink in the ink channel to generate a bubble. The expanding bubble ejects ink from the nozzle whereupon the bubble rapidly collapses, allowing generation of a subsequent bubble.
Japanese Patent Application Kokai No. SHO-59-26270 and Japanese Patent Application Kokai No. SHO-61-69467 describe another ink jet printer which ejects droplets by pressure generated from an expanding bubble. Japanese Patent Application Kokai No. SHO-61-69467 describes a sealed pressure chamber type ejector with a sealed chamber filled with a pressurizing liquid (a liquid which generates a bubble when heated to its boiling point by a thermal pulse, the expansion of the bubble increasing the pressure in the sealed chamber). One surface of the sealed chamber is formed from a resilient pressure transfer plate. A bubble is generated in the pressurizing liquid in response to a pulse of heat. The pressure transfer plate deforms under the increase in pressure in the sealed chamber. The deformation of the pressure transfer plate pressurizes the ink in the ink channel and ejects it from a nozzle. Further, the pressurizing liquid filling the pressure chamber is a liquid with low boiling point such as an alcohol-based, water-based, or organic-solvent-based liquid.
Unlike bubble ink jet apparatuses which heat the ink, sealed pressure chamber type ejectors and piezoelectric type ejectors pressurize the ink, and can eject even heat sensitive liquids, as long as they are not too viscous. However, only piezoelectric type ejectors are conventionally produced for ejecting hot-melt ink. Hot-melt ink is solid at room temperatures, and therefore must be heated and melted before being ejected as a liquid. Japanese Patent Application Kokai No. HEI 2-111549 describes a sealed pressure chamber type ejector for ejecting hot-melt ink, and further describes that water forms a good pressurizing liquid, but an actual apparatus has yet to be produced.
No concrete structures or specific methods of operation are provided in the art for examples of sealed pressure chamber type printers, such as those described in Japanese Patent Application Nos. SHO-59-26270 which corresponds to U.S. Pat. No. 4,480,259, SHO-61-69467, and HEI-2-111549. Also, no practical examples have been announced. When determining the melting point of hot-melt ink to be used in a printer, two points must be taken into consideration. On the one hand, the higher the boiling point, the better the characters printed on recording paper will wear in storage. On the other hand, the lower the boiling point of hot-melt ink, the easier the hot-melt ink is to melt. Hot-melt ink with melting point of 60.degree. C. is commonly used as a compromise between these two requirements. All piezoelectric ink jet printers produced today maintain at least the entire ink channel between 130.degree. and 150.degree. C. to lower the viscosity of the liquid hot-melt ink to the range of 10 to 20 mpa or lower as required for proper ejection.
A sealed pressure chamber type ejector must also meet the same conditions to eject hot-melt ink. That is, the temperature at the ink side of the pressure transfer plate must be 130.degree. to 150.degree. C. Further, the pressure transfer plate must have good resilience. It must be a metal thin film or thin film of a heat-resistant resin from several .mu.ms to several 10's of .mu.ms thick. Also, the temperature of the pressure chamber sealed on one side by the pressure transfer plate must be maintained at a temperature near that of the liquid hot-melt ink.
Japanese Patent Application No. HEI-2-111549 recommends that water form the pressurizing liquid in the sealed pressure chamber type hot-melt ink jet printer described therein. However, unless some type of heat isolation is provided at the pressure transfer plate, maintaining the hot-melt ink at 130.degree. to 150.degree. C. would raise the temperature of the water in the pressure chamber to near the same temperature, raising the pressure of the water to three to five atmospheres. Consequently, the pressure transfer plate would normally be expanded into the ink channel. While the pressure transfer plate is in this expanded condition, bubble pressure cannot increase the pressure in the sealed pressure chamber sufficiently for ejecting the ink.